The remains of a fertilizer plant blaze after an explosion in the town of West, near Waco, Texas (Reuters)

Texas police said the cause of a fire that triggered a massive blast at a Texas fertiliser plant is yet unknown but criminal activity can't be ruled out.

The West Fertilizer plant, 18 miles north of Waco, Texas was hit by a massive explosion at about 19:50 (00:50 GMT). The blast was heard as far as 45 miles (72km) away.

"At this point we don't know...whether it's some criminal activity or fire that sparked from some kind of chemical reaction," Sergeant William Swanton of the Waco police department told a press conference.

Swanton confirmed a number of people died in the massive blast, but couldn't give a casualty figure.He later estimated five to 15 people were killed.

Swanton said that site of the explosion is being treated as a "crime scene" even though there is no indication of criminal activity so far.

"Right now we have a tremendous amount of injuries, probably over 100 injuries at this time," D.L. Wilson, from the Texas Department of Public Safety, told a press conference."

Police said a fire started at the plant at about 6pm local time (11:00 GMT).

A 2.1-magnitude earthquake was recorded in the area as the explosion went off, the US Geological Survey reported.

"The force of the blast was transmitted as shockwaves through the ground and picked up by instruments as if it was an earthquake," explained Roger Musson, from the British Geological Survey.

The explosion has occurred two days ahead of the 20th anniversary of the Waco massacre.

A 50-day-siege by US security forces at a ranch property of the Branch Davidians religious sect ended in carnage as a fire destroyed the compound. More than 70 people died.

US Representative Bill Flores, whose district includes West, said he doubts foulplay was behind the incident.

"I would not expect sabotage by any stretch of the imagination," he told CNN.

The blast has been compared to a "nuclear bomb" explosion by West Mayor Tommy Muska.

It was a like a nuclear bomb went off," Muska told the CNN, "a big old mushroom cloud."

"There are a lot of people that will not be here tomorrow," Muska added.

The explosion has destroyed several buildings in the vicinity of the factory, including a nursing home, trapping the residents. Some firefighters who rushed to the scene of the fire emergency are also believed to be among the missing.

Local TV images showed an apartment complex completely razed by the blast.

City council member, Al Vanek, said there was a four-block area around the explosion "that is totally decimated."